The present invention relates to an armature of a rotary electric machine using U-shaped upper and lower coil conductors joined for forming an armature coil and, more particularly, to an armature of a rotary electric machine in which joined parts of upper and lower coil conductors are assuredly protected from centrifugal force and insulated from circumferentially adjacent joined parts.
An armature for a commutator-type rotary electric machine is known as disclosed in JP-A 7-231618. This armature includes, as shown in FIG. 15, lower coil conductors 200 and upper coil conductors 300 assembled to an armature core 100.
The coil conductors 200 and 300 comprise, respectively, coil trunks 210 and 310 accommodated in slots 110 of the armature core 100, coil arms 220 and 320 formed to extend from both axial ends of the coil trunks 210 and 310 radially inwardly toward a shaft in parallel with the axial side end faces of the armature core 100, and coil extensions 230 and 330 formed to extend from the radially innermost peripheral ends of the coil arms 220 and 320 toward the axially outer side. The tip ends of the coil extensions 230 and 330 aligned radially are electrically joined by welding or the like to provide an armature coil. As this armature uses one coil arm 320 of the upper coil conductor 300 as a commutator segment, a commutator need not be provided separately and resistance of the armature coil against centrifugal force can be enhanced. Thus, the armature can be used in high speed rotation environment. As the speed reduction ratio can be set larger in case of a starter having a speed reduction mechanism for an automotive vehicle, the armature contributes to the reduction of the starter in size.
In each of the coil conductors 200 and 300 used in the above armature, however, the volume (heat capacity) of the coil arms 220 and 320 is larger than the volume (heat capacity) of the coil extensions 230 and 330. Therefore, when the tip ends of the coil extensions 230 and 330 are joined by welding, the welding heat dissipates from the coil extensions 230 and 330 to the coil arms 220 and 320. This results in, as shown in FIG. 16, imperfect joint part A at a joint between the radially outer face of the lower coil extension 230 and the radially inner face of the outer coil extension 330. This imperfect joint part A means, from the standpoint of fracture mechanics, that a crack exists initially at a boundary with the joined part. Therefore, it is likely to occur that the crack progressively extends from the imperfect joint part A to cause a crack in the joined part (i.e., from left to right in FIG. 16) as well due to the centrifugal force, when the armature rotates at a high speed.
For obviating the imperfect joint part A, the welding heat may be applied until the coil extensions 230 and 330 melt sufficiently or the axial length L of the coil extensions 230 and 330 may be shortened as shown in FIG. 17. However, the coil extensions 230 and 330 will melt excessively causing the conductor material melted by the welding heat to extend in the circumferential direction. As a result, as shown in FIG. 18, joined parts 400 which are located circumferentially adjacently will come into contact with each other disabling assured electrical insulation therebetween.